1. Field of the Invention
The present invention relates to a control apparatus of an alternating-current (AC) motor operating in a discrete-time fashion.
2. Description of the Related Art
When achieving a positioning control in a control apparatus of a permanent-magnet (PM) alternating-current motor, an angle of a rotor of the motor is required to be sensed for a feedback of information of the rotor angle. For this purpose, the conventional technology has employed a sensor such as an encoder or a resolver. Furthermore, in the alternating-current motor, it is necessary to change a phase of a current flowing through each winding of the motor depending on the rotation angle of the rotor. In order to sense the magnetic pole positions of the rotor, the conventional system employs a sensor such as a pole sensor.
However, these sensors above cannot be used in general at a high temperature and is moreover not satisfactorily resistive against vibration and shock. In consequence, the conventional motor control apparatus using such sensors has been attended with a problem that the desired control operation cannot be achieved in such an environment.
On the other hand, a state estimation observer has been proposed by Lawrence A. Jones et al. in "A STATE OBSERVER FOR THE PERMANENT-MAGNET SYNCHRONOUS MOTOR", IECON 1987 Conference, Cambridge, Mass., Nov. 2-6, 1987. In this observer, a direct quadrature (dq) transformation model of a permanent-magnet alternating-current motor and a linear observer theory are applied to estimate a rotor angle and an angular velocity thereof from winding current and voltage values of the alternating-current motor. Furthermore, advantages associated with the direct quadrature transformation model has been described in the article.
However, in an actual case where a control apparatus is configured with a state estimation observer to control an alternating-current motor, if the state estimation observer has a fixed gain, there have been problems that an estimation value produced by the observer becomes unstable when a speed of the motor is changed in a wide range and that the response time to obtain the observer estimation value becomes to be longer. Namely, the alternating-current motor cannot be controlled in a stable state at a high speed. For example, in a case where the observer gain of such a motor control system is set to a value suitable for a speed substantially equal to 1,500 rotations per minutes (rpm), if the control apparatus attempts to control an alternating-current motor rotating at 500 rpm, the state estimation observer cannot obtain a converged estimation value.
Furthermore, the rotation angle estimated by the state estimation observer is an electric angle, which leads to a problem that assuming the number of rotor poles to be N (.gtoreq.2), a unique mechanical angle cannot be determined.
In addition, the state estimation observer receives as inputs thereto winding voltages and currents of the alternating-current motor, which results in a problem that when a phase difference is found in association with the currents and the voltages, a rotation angle and an angular velocity thus estimated include estimation errors.